1. Something you should come up with when your lame excuse does not take expected effect;

an additional lie intended to make your original lie sound more convincing;

2. A crucial element in the chain of one's lie.

Guy: You're so special, I like you so much!
Girl: That's so sweet. What particulalrly do you like about me?
Guy (to himself): Damn! I should have come up with a backup lie before I told her that!

Teacher: Where is your homework?
Student: My dog ate it.
Teacher: Why didn't you give him the laxative?
Student: Um...
Teacher: Didn't even bother to make up any backup lie... You get an F, you, moron!